16.9 Snake fungal disease

Beginning in 2006, severe skin infections were reported along with a rapid
decline in a timber (Crotalus horridus) population in the northeastern
USA. In 2008, similar infections with a possible fungal cause emerged in
Illinois, USA in an endangered population of Massasauga rattlesnakes (Sistrurus
catenatus). This infectious disease became known as
Snake
Fungal
Disease (SFD),
and by 2015 SFD had been documented in most of the eastern USA. SFD is a major
conservation concern in North America because it has the potential to cause
lethal infections and contribute to extinction of localised snake populations.

SFD is caused by Ophidiomyces ophiodiicola, a keratinophilic fungus
that is widely distributed in eastern North America. It has a broad host range
and is the predominant cause of fungal skin infections in wild snakes. It often
causes mild infections in snakes coming out of hibernation and is an emerging
pathogen of captive snakes in North America. SFD has been documented in 23
species of snakes, although this is likely to be an underestimate of the number
of susceptible species. SFD hosts are both phylogenetically and ecologically
very different, which indicates that other species of snakes in the US might now
be infected or susceptible to SFD (Lorch et al., 2016; Burbrink et
al., 2017).

Ophidiomyces ophiodiicola has been isolated from captive snakes
outside North America, but the pathogen had not been reported from wild snakes
elsewhere until carcasses and moulted skins from wild snakes collected during
2010-2016 in Great Britain and the Czech Republic were screened for the presence
of skin lesions, and for O. ophiodiicola using PCR DNA detection. The
fungus was detected in 8.6% of the specimens and further analysis confirmed that
Ophidiomyces-SFD occurs in wild European snakes. Phylogenetic analyses
indicated that Ophidiomyces isolated from European wild snakes belonged
to a clade distinct from the North American isolates; so, the European and North
American diseases involve different strains of Ophidiomyces ophiodiicola
(Franklinos et al., 2017).

Potentially, therefore, Ophidiomyces ophiodiicola is a global
emerging fungal pathogen of reptiles in the wild. We’re talking about
reptiles and potential extinctions here, so this is about the right place to
remind you about dinosaurs. They are mostly dead. Mostly, because all those
birds that are flapping around out there are the remnants of a group of
reptiles that once ruled the Earth for several hundred million years; until
they went extinct. Taking advantage of the wide-scale extinction events of
other organisms is an often-repeated feature of fungal evolution. The period
800 to 600 million years ago featured successive virtually global
glaciations (snowball Earth episodes). The Permian-Triassic (P-Tr)
extinction event that occurred approximately 251 million years ago (known as
the Great Dying and the Earth’s most severe extinction event so far) was
studied by Visscher
et al. (1996) who summed it up with the quotation:

Much the same is true for the Cretaceous-Tertiary (K-T) extinction of 65
million years ago, the result of a meteor collision that caused the
Chicxulub crater in Mexico, which is blamed for the extinction of the
dinosaurs. There was also widespread deforestation right at the end of the
Cretaceous, which is assumed to be due to post-impact conditions. However,
coincident with all this death and destruction of animal and plant life at
the K-T boundary there is a massive proliferation of fungal fossils:

‘…This fungi-rich interval implies wholesale dieback
of photosynthetic vegetation at the K-T boundary in this region. The fungal
peak is interpreted to represent a dramatic increase in the available
substrates for [saprotrophic] organisms (which are not dependent on
photosynthesis) provided by global forest dieback after the Chicxulub
impact...’ (Vajda & McLoughlin, 2004).

So, it is the same story as at the other extinction boundaries: while the
rest of the world was dying, the fungi were having a party!

But that Chicxulub meteor might not have had the last word on dinosaur
extinction, because the massive increase in the number of fungal spores in
the atmosphere of the time may have caused fungal diseases that

‘…could have contributed to the demise of dinosaurs
and the flourishing of mammalian species…’ (Casadevall, 2005).

A reminder, perhaps, that the fungi started the eukaryote journey by
spring-cleaning the early Earth, and they’ve been cleaning up and modifying
the planet and its biosphere ever since (Moore, 2013).